The present invention relates to a power transmitting device.
Power transmitting devices of known configuration include a type that has a hollow cylindrical first rotor, which receives driving force and rotates, and a shaft-like second rotor, which is coaxial with and rotatably arranged in the first rotor. A clutch mechanism is located between the first rotor and the second rotor, and selectively permits and shuts off torque transmission between the first rotor and the second rotor. The clutch mechanism includes, for example, a main clutch, a pilot clutch, and a cam mechanism. The main clutch permits the torque transmission between the first rotor and the second rotor when being pressed along the axial direction of the first rotor. The pilot clutch is arranged adjacent to the main clutch along the axial direction of the first rotor. In accordance with the operation of the pilot clutch, the cam mechanism is selectively switched between a state where it presses the main clutch along the axial direction of the first rotor and a state where it does not press the main clutch.
For example, Japanese Laid-Open Patent Publication No. 2003-14001 discloses a power transmitting device having an electromagnetic clutch as a pilot clutch. The electromagnetic clutch has clutch plates that frictionally engage with each other when pressed by an armature that is moved when attracted by an electromagnet. When the pilot clutch is engaged, the torque due to the difference in rotation between the first rotor and the second rotor is converted into a linear motion of a cam member (the main cam) along the axial direction of the first rotor. Accordingly, the main clutch is pressed along the axial direction of the first rotor.
Although the cam member is located farther from the electromagnet than the armature with respect to the axial direction of the first rotor, the cam member can be magnetized if leakage of magnetic flux occurs. In such a case, the armature is attracted by the magnetized cam member. This hinders the movement of the armature toward the clutch plates, that is, the smooth operation of the electromagnetic clutch. The responsiveness and controllability of the power transmitting device are therefore degraded.
Particularly, the armature can be moved freely along the axial direction of the first rotor when the electromagnet is not energized. Therefore, when the electromagnet is not energized, the armature can be attracted by the magnetized cam member and separate from the pilot clutch. Even if the electromagnet is energized in this state, the armature may not be attracted by the electromagnet toward the pilot clutch.
In this respect, a typical first rotor accommodating a pilot clutch and a cam mechanism is made of a non-magnetic material such as an aluminum alloy to prevent magnetic flux leakage from occurring. Further, Japanese Laid-Open Patent Publication No. 2002-48157 discloses a configuration in which a spacer made of a non-magnetic material is inserted between an armature and a cam member, so that a space exists between the armature and the cam member. Also, Japanese Laid-Open Patent Publication No. 2002-61677 discloses a configuration in which a surface of an armature that does not face an electromagnet is coated with a non-magnetic material. However, these configurations unexpectedly complicate the manufacture of power transmitting devices and thus increase the manufacturing costs. There is thus still room for improvement.